1. Field of the Invention
The present invention relates to telecommunications systems in general and in particular to wireless mobile telecommunications systems.
2. Description of the Prior Art
Creating reliable telecommunications systems presents significant challenges to designers and engineers. Designers of mobile telecommunications systems, in particular, face the formidable challenge of balancing cost, size, weight, features, and performance in these increasingly advanced systems. As newer standards emerge, such as the Personal Communication Systems (PCS) standard, the need to enhance the basic reliability of these mobile systems remains ever present.
In wireless telecommunications system design, a transmitter emits a RF signal usually termed a xe2x80x9ccarrier signalxe2x80x9d that is modulated in a predetermined fashion in accordance with the sequence of information being transmitted. At the receiver, this modulated RF signal is received and processed to recover the transmitted information. In reality, the signal received by the receiver is a composite of the originally transmitted signal and unwanted interference and methods exist to mitigate such interference. Various techniques of carrier signal modulation are known and are generally chosen based on power and spectral efficiency, as well as their relative vulnerability to interference. Commonly adopted modulation techniques include frequency modulation (FM), amplitude modulation (AM), and phase modulation (PM). Of course, numerous implementations exist within each general modulation technique, and some implementations embody aspects from two or more of the general techniques.
Mobile telecommunications systems employing phase modulation techniques change or modulate the phase angle of the transmitted carrier signal to convey the information being transmitted. Typically, the stream of data to be transmitted is digitized and encoded, often with error-correction information added, into a stream of multi-bit symbols, with each symbol having a unique and predetermined associated phase angle. The transmitter then modulates its carrier signal in accordance with the phase angle corresponding to each transmitted symbol at a predetermined symbol rate. At the receiver, this phase modulated signal is received and processed to recover the transmitted symbols, which can then be decoded to recover the original information. The process of recovering the phase-modulated symbol information from the received RF signal is termed demodulation and the receiver may employ either coherent or non-coherent demodulation techniques.
In coherent demodulation, the receiver attempts to phase-compensate the received signal to minimize errors in the demodulated signal. A receiver using coherent demodulation can compensate the phase of the received RF signal based on initial estimates of phase and frequency and then adjust that compensation based on measured phase error in the decoded symbols. A receiver using coherent demodulation techniques can demodulate the received RF signal based on the absolute phase angle value of each received signal, or based on the difference in phase angles between two consecutively received symbols, which is termed xe2x80x9cdifferential detection.xe2x80x9d Non-coherent demodulation is based on differential detection techniques as described above and does not phase-compensate the received signal to minimize errors.
Certain reception conditions favor the use of coherent demodulation versus non-coherent demodulation. Under such conditions, a receiver employing coherent demodulation techniques will have superior performance as compared to an otherwise comparable receiver employing non-coherent demodulation. Here, superior performance is defined as a lower bit error rate (BER) in the information recovered from the received RF signal. However, there exist a range of reception conditions under which a receiver employing non-coherent demodulation will exhibit a lower BER as compared to an otherwise comparable receiver using coherent demodulation.
In the existing art, mobile telecommunications systems are designed to include receivers that fixedly operate using only one of the two techniques, either coherent or non-coherent demodulation. By not adapting the receivers"" demodulation technique in accordance with changing reception conditions, the prior art receivers do not operate with the optimal demodulation technique in all circumstances. Accordingly, there remains a need for a mobile telecommunications system having a receiver designed to receive phase-modulated radio signals that can adapt its demodulation technique from coherent to non-coherent operation in response to detected changes in reception conditions, so as to minimize the BER in the received data.
The present invention fulfills this need in the art by providing a method and apparatus for a mobile telecommunications receiver to operate using an adaptive demodulation scheme. In the present invention, the receiver can operate using fully coherent demodulation, non-coherent demodulation, or xe2x80x9cpartially coherentxe2x80x9d demodulation wherein only a portion of the phase compensation signal normally used in coherent demodulation is applied to the received signal. The receiver according to the present invention can progressively reduce the phase-coherent compensation applied to the received signal in response to changing reception conditions, as indicated by changes in the magnitude of detected phase errors. Increases in the magnitude of phase errors detected in the demodulated signal correspond to increasing inaccuracies in the receiver""s phase estimates used to phase compensate the received signal. In the present invention, the receiver using adaptive demodulation fractionally scales or weights its phase compensation signal so that less and less phase compensation is applied to the received signal as the magnitude of detected phase error increases. Beyond a given threshold of detected phase error, the receiver operating with adaptive demodulation in accordance with the present invention, reduces the phase compensation applied to the received signal to zero and operates with fully non-coherent demodulation, as is well understood by those skilled in the art.